This invention relates, in general, to an apparatus for forming pliable material into rod-shaped pellets and, more particularly, to an apparatus for compacting and extruding material into elongated strands or rod-shaped elements of either predetermined length or of an indeterminate length.
Formation of diverse materials, such as vegetable matter, slaughterhouse waste, and the like, into rod-shaped elements of various selected lengths is a common practice employed in processing of materials. The objective of forming the materials into rod-shaped elements, which are commonly referred to as "pellets" and the process as extruding or "pelletizing," is to place the material into a form that can be easily handled and, if appropriate, subjected to further processing, such as drying operations. Principally, pelletizing techniques are utilized to place a material which is initially in a granular or particulate form into a compacted mass having a particular shape and having a desired structural integrity so as to maintain its configuration when either being utilized or subjected to further processing.
For example, animal and pet foods typically are sold in a pellet form. This form makes the food relatively easy to handle, uniform in texture and consistency, and typically reduces spoilage as compared to non-pelletized foods such as raw grain, grasses, and animal products. Transforming such foods into pelletized form generally sterilizes the food and removes much of its moisture making it less subject to degrading and spoilage.
Generally, extruders are used for pelletizing of materials. Typical extruders use an auger in a barrel to displace material along the axis of the barrel and to force the material through a perforated plate at the end of the barrel, in much the same way as common meat grinders work. The perforated plate is positioned in axial relationship to a helical auger conveyor which, when revolved, will axially displace the material against the extrusion plate and force the material through the perforations. Those perforations are of a size that is appropriate for the particular material and the desired pellet configurations so that, when the material is extruded, it will be compacted into a mass having adequate structural integrity to maintain its configuration. The material extruded initially may be in the form of elongated strands, but those strands generally are separated after extrusion into rod-shaped elements that are of a desired length.
The extruding auger is mechanically coupled to a driving mechanism such as an electric motor and is operated at a desired rotational speed with sufficient power input to cause displacement of the material along the axis of the auger and to be forcefully extruded through the extrusion plate. This auger-type extrusion mechanism requires a substantial amount of power to extrude the material through the extruding plate. The high levels of mechanical power required are a result of the high pressure-type extrusion systems utilized in these prior art mechanisms as well as the configuration of those devices. Basically, the augers in these prior art mechanisms are designed to build pressure in the material along the entire length of the auger. This allows the material to be compressed to a very high pressure, e.g., 300-500 psi, by the time the material reaches the extrusion plate. Typically, such high pressure extruders are operated with a relatively dry mix and materials, generally about 10 to 25 percent by weight moisture content. These extruders commonly use steam injection to heat and lubricate the mixture in the extruders. In the case of food materials, these high pressure extruders typically impart sufficient energy input to partially cook the material involved. However this high power required in prior art extruders make them expensive to buy, operate, and maintain. Large motors are required to produce the high pressures involved. For example, it is common for a commercial extruder to require a 150 to 800 horsepower drive motor.
A variation on the meat grinder approach includes use of a cylindrical extruding section. These extruders operate in fundamentally the same manner as other prior art extruders, utilizing a pressure buildup along the length of the auger to force the material out through the extrusion holes in the barrel. Such a system is described in U.S. Pat. No. 5,242,292, which is incorporated herein by reference. Thus, prior art barrel extruders are subject to generally the same limitations as other prior art extruders, requiring high power inputs and relatively dry material for effective extrusion.
It is common in both types of prior art extruders to use an extruding auger to build pressure and a separate wiper mechanism to facilitate pushing of material through extrusion orifices. For example in the U.S. Pat. No. 5,242,292 , a pressurizing auger, see item 44 FIG. 5., is used to subject the material to increasing shear and pressure along the barrel. The material is thus moved axially along the extruder barrel, building extrusion pressure. After the extrusion operation, a separate wiper vane, see item 70 on FIG. 4 and Col. 4, line 39, is used to shear material at the extrusion orifices, 58. This wiper vane is not designed to effect axial movement of material.
Prior art extrusion mechanisms have not been found to be capable of efficiently and effectively pelletizing material with high moisture content, such as in food processing or processing of newly-harvested crops. High pressure extrusion of high moisture materials causes loss of pellet form due to high discharge velocities created through the extrusion holes, effectively limiting moisture content in material to be extruded to a maximum of about 30 percent by weight. One means of addressing this problem has been to partially dry and, in the case of food, cook the material prior to extrusion. However, this requires that additional material preparation steps and apparatus be included in the overall process, increasing complexity and cost of the overall pelletizing operation.
The high power inputs required to operate the prior art extruders requires the mechanisms to have extremely large drive shafts to handle the high torque requirements. Further, the high pressure and power levels involved also work to cause degradation in the auger mechanisms through erosion and stress. These factors lead to increased maintenance and operating expense. Moreover, such prior art apparatus has the detrimental aspect of being relatively slow in operation and, thus, being capable only of processing relatively small quantities of material in specified time periods.
An additional limitation of such prior art apparatus is that the extruding auger is difficult to manufacture. Due to the high pressures involved, extremely heavy and rigid auger rotors are required. Fabrication of such rotors is difficult and further adds to the complexity of such devices.